In many cases, small auxiliary power unit gas turbines are required to start at very high altitudes, e.g., on the order of 40,000 feet or more. It is known that large main propulsion gas turbines cannot start at such high altitudes and, therefore, this requirement for small auxiliary power unit gas turbines is most demanding, particularly since it is generally accepted that combustion is very difficult at best at such altitudes and even more so in small scale applications. Furthermore, small auxiliary power unit gas turbines cannot afford the sophisticated fuel injector and combustor designs customary in large main propulsion turbines.
Generally speaking, the controlling elements for combustion are adequate fuel atomization, adequate volume for mixing, and adequate volume for reaction. It is also known that the time available for fuel evaporation is proportionate to the fuel droplet size which means that, with a small scale combustor, it is important to have a proportionate improvement, i.e., reduction, in the fuel droplet size. However, with combustor scale reductions, the fuel atomization is typically worsened primarily because of enhanced viscous small scale effects which are commonly experienced.
In recent years, there have been a number of improvements for providing a considerable reduction in fuel droplet size. For instance, for a given fuel pressure, impingement fuel pressure atomization provides a marked improvement in fuel droplet size over the commonly used swirl pressure atomizing fuel injectors. By also reducing the number of fuel metering orifices, it is possible to attain improved reliability, lowered cost, and improved ignition.
As for mixing and reaction, it is known that the air and fuel should be uniformly mixed upon entering the combustor. Further, it is known that adequate volume must be available to complete reaction and, since size and pressure are inversely related, the larger the combustor scale, the lower the pressure at which combustion can be achieved. As a result, it is known that larger engines should ordinarily have a higher altitude capability than small engines.
In order to overcome the problems associated with smaller combustors, it has been necessary to provide advanced techniques for high altitude starting of auxiliary power units. It has, thus, been a goal to be able to provide a turbine engine, particularly a small auxiliary power unit, which is fully capable of very high altitude starts but with simple, inexpensive design characteristics. In furtherance of the objective of achieving this goal, the present invention is directed to a high altitude turbine engine starting system having truly unique characteristics